Name the denominator
Write price currency, price per pack or per item, items per pack and the unit of the stock quantity. A source quantity of 5 could mean five packs or five items. Do not infer units from a description, a small price or a familiar product. The W3C tabular model explicitly treats units as metadata rather than automatically interpreting every number.
- Hold rows with unknown pack size or price basis.
- Keep pack sizes positive whole numbers only where that matches the actual product rule.
- Weight-based goods, variable packs and currency conversion need a different contract.
Comparison price is not the payable amount
The worked example is USD 19.99 per pack of 12, ordering three packs. That means 36 items and a pack-based amount of USD 59.97. A displayed per-item value rounded half-up to two decimals is USD 1.67; multiplying that display by 36 gives USD 60.12, not the pack-based amount. The example's rounding policy is explicit and is not tax or accounting advice.
- Calculate the contractual amount from its actual price basis.
- Keep sufficient intermediate precision and round at the owner-approved boundary.
- Decimal arithmetic does not decide the unit, tax, freight or rounding policy for you.
Acceptance checks dimensions and amounts
Use an invented pack offer and assert both item quantity and pack-based amount. Include a missing pack size and a zero pack size; under this example policy both are exceptions. Compare a whole-item offer only after confirming compatible currency and units. The provided arithmetic is synthetic and excludes tax, freight, discounts and purchase-order submission.
- A neatly formatted two-decimal display is not evidence that the calculation used the right basis.
- No supplier order or invoice was created.
Priced enquiry and non-fit
One existing-code unit-conversion defect may fit the synthetic regression-test repair, from £295 after bounded reproduction and a fixed quote. One new unit-aware comparison interaction may fit the existing web-app feature-preview offer, from £750, if its isolated private preview and other eligibility conditions already hold. Tax policy, landed-cost modelling, payments, accounting changes and real purchasing actions are excluded. Send invented pack price, size, quantity and expected result initially; prices are untested and payment follows agreed checks and sign-off.
Sources and limits
- W3C tabular model: units metadata Checked 2026-10-11.
- Units are not a built-in part of the tabular model and can be expressed through metadata.
- Python decimal: exact decimal inputs and explicit rounding Checked 2026-10-11.
- Constructing Decimal from a float preserves the float's exact value, rather than recovering the originally intended decimal.
- quantize rounds to a fixed exponent with the selected rounding policy.
- Existing repair offer Checked 2026-10-11.
- Existing preview feature offer Checked 2026-10-11.